Laterite ores are potentially the world's largest source of nickel and cobalt. In general, most deposits of nickel/cobalt laterites contain three major zones based on morphology, mineralogy and chemical composition. These three zones, from the base to the surface, atop weathered parent bedrock materials are the saprolite zone, the transition zone and the limonite zone. There is generally a large variation in total thickness of the laterite deposit, as well as individual zone thickness.
The saprolite zone consists predominantly of “saprolitic serpentine” minerals and a large variety of nickel/magnesium silicate minerals. The weathering process, or “serpentinization” of the parent bedrock material is characterised by a decrease in the magnesium content and an increase in the iron content of the top layer of ore body. The resulting saprolite zone contains between 0.5% and 4% nickel and a higher magnesium content, which is normally over 6% wt.
The not well defined transition zone is composed essentially of limonite and saprolite. It also commonly contains nickel in the range of from 1.0% to 3.0% with co-existing cobalt ranging from 0.08% up to 1% (associated with asbolane, a hydrated manganese oxide).
The limonite zone, located on the top zone of the lateritic ore body, contains nickel ranging from about 0.5% to 1.8% and consists of goethite-rich and/or hematite-rich ore, which is rich in iron and cobalt. It has a lower magnesium content than saprolitic type ore. Due to stronger weathering, limonitic ore contains dominantly fine and soft particles of goethite and/or hematite. Sometimes the weathering has not been fully completed and either the hematite or the goethite rich sections are not present. Alternatively, depending upon the climatic condition, the limonite zone will still contain residual iron/aluminium silicates, such as nickel-containing smectite, nontronite and chlorite. At atmospheric pressure and ambient temperature, the acidic leach of limonite is slow. The whole-ore dissolution reaction using sulfuric acid is shown as follows:
Limonite Leach(Fe,Ni)O.OH+H2SO4→NiSO4+Fe+3+SO4+2+H2O  Eq. 1Goethite(Fe,Ni)2O3+H2SO4→NiSO4+Fe+3+SO4+2+H2O  Eq. 2Hematite
The iron content of limonite ore is normally in the range of 25-45% wt which corresponds to 40-72% wt goethite (FeOOH) or 36-64% wt hematite (Fe2O3).
Consequently the dissolution of Ni-containing goethite or hematite of a limonitic heap causes the instability of a heap, such as severe volumic slumping or shrinkage, and poor irrigation permeability.
The less-weathered, coarse, siliceous and higher nickel content saprolites tend to be commercially treated by a pyrometallurgical process involving roasting and electrical smelting techniques to produce ferro nickel. The power requirements and high iron to nickel ore ratio for the lower nickel content limonite blends make this processing route too expensive. Limonite ores are normally commercially treated by a combination of pyrometallurgical and hydrometallurgical processes, such as the High Pressure Acid Leach (HPAL) process, or the reduction roast—ammonium carbonate leach process.
Acid leaching of saprolitic ore is rarely practised commercially for the reason that a widely applicable process has not been developed for recovering the nickel from the leach solution in an economical and simple manner.
While heap leaching copper ores is well known as a commercial operation, there are several differences between heap leaching of copper containing ores that also contain some clay components, and the lateritic ores that have substantial fine and/or clay components. In addition, the acid consumption of laterite ore is ten-fold that of heap leaching copper ores.
Heap leaching of nickeliferous oxidic ore has been proposed in recovery processes for nickel and cobalt and is described, for example in U.S. Pat. Nos. 5,571,308 and 6,312,500, both in the name of BHP Minerals International Inc.
U.S. Pat. No. 5,571,308 describes a process for heap leaching of high magnesium containing laterite ore such as saprolite. The patent points out that the fine saprolite exhibits poor permeability, and as a solution to this, pelletisation or agglomeration of the ore is necessary to ensure distribution of the leach solution through the heap.
U.S. Pat. No. 6,312,500 also describes a process for heap leaching of laterites to recover nickel, which is particularly effective for ores that have a significant clay component (greater than 10% by weight). This process includes sizing of the ore where necessary, forming pellets by contacting the ore with a lixiviant, and agglomerating. The pellets are formed into a heap and leached with sulfuric acid to extract the metal values. Sulfuric acid fortified seawater may be used as the leach solution.
International application PCT/AU2006/000606 (in the name of BHP Billiton SSM Technology Pty Ltd) also describes a process where nickeliferous oxidic ore is heap leached using an acid supplemented hypersaline water as the lixiviant with a total dissolved solids concentration greater than 30 g/L in order to leach the heap.
Heap leaching laterites offers the promise of a low capital cost process, eliminating the need for expensive and high maintenance, high pressure equipment required for conventional high pressure acid leach processes. These patents and applications exclude the processing of limonitic laterite for heap leach because, in addition to the low reactivity, the reaction mechanism of whole-ore dissolution shown in Eq. 1 and 2 may lead to the collapse and/or poor permeability of the heap due to the dissolution of nickel containing goethite or hematite as outlined above.
Heap leaching of laterite nickel ore results in a solution containing nickel plus impurities such as iron and aluminium. Conventional processing requires that the iron and aluminium be precipitated before the nickel and cobalt are recovered. Typically, iron and aluminium are precipitated using limestone. This results in a slurry of iron and aluminium hydroxides together with gypsum.
The cost of limestone is not large, however it is an operating cost. The sulfate content of the gypsum by-product is derived from the original sulfuric acid added to the process. As the price of sulfur rises, better use of this acid content would reduce operating costs. Thus replacing the limestone for the neutralisation duty has potential to save both the cost of the acid equivalent to the gypsum, and also the cost of the limestone used for the precipitation process.
The present invention aims to provide a process where a saprolite ore may be used to neutralise the PLS from a heap leach process.
It is a desired feature of the present invention to provide a process where iron and/or aluminium impurities are effectively precipitated from the PLS without operating loss of sulfur through gypsum precipitation.
It is a further desired feature to better utilise the acid that is generated during the hydrolysis and precipitation of iron and/or aluminium, maximising the use of acid in the recovery of nickel and/or cobalt.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.